This invention relates generally to an optical film and a process for manufacture thereof, and more particularly to processes in which the properties of materials in the film are exploited to control the response of the materials during processing.
Over the past several years the use of polymeric films to produce high-quality optical films has been explored. The investigated polymeric optical films typically exploit differing indices of refraction between layers of a multilayer film. For example, a multilayer optical film may be constructed by alternating polymer layers having high indices of refraction with polymer layers having low indices of refraction. Where two adjacent layers are formed with a relatively large mismatch between the respective indices of refraction, light of a particular wavelength is reflected at the interface of the two layers. The wavelength of light reflected is dependent on the optical thickness of the adjacent layers. Optical films may also be made of polymeric blends exploiting relative refractive indices relationships of different phases of the blend.
One type of multilayer optical film uses birefringent polymers to produce the mismatch between adjacent indices of refraction. In such films, the multilayer film is drawn either uniaxially in a direction along only one in-plane axis of the film or biaxially in a direction along two in-plane orthogonal axes of the film. As a result of drawing, molecules within one of the two adjacent layers become oriented in the direction of the draw. The molecular orientation alters the index of refraction of the affected layer in the draw direction.
Drawing a multilayer film, where one of the layers of the film is birefringent, can be used to obtain a mismatch of refractive indices of two adjacent layers in the draw direction. If the refractive indices of the two layers in the non-drawn in-plane direction match, the multilayer film may be used to reflect light of one polarization and transmit light of another. Such a film may be employed, for example, as a reflective polarizer. If a multilayer film is drawn such that the affected layer is oriented in the direction of two orthogonal in-plane axes, a mismatch in both directions may be obtained. Such a film may be used to reflect light of both polarizations (e.g., a mirror for the respective wavelength of light).
As noted above, one or more of the materials used in polymeric optical films has optical characteristics which change as the material is drawn. These characteristics tend to assume a directional orientation which depends on the direction in which the material is drawn. Such optical films may be described in terms of three mutually orthogonal axes, namely, two in-plane axes x and y and a thickness axis z. The optical properties of such optical films depend generally on the indices of refraction n.sub.x, n.sub.y and n.sub.z of each material in the direction of the x, y and z axes, respectively. Thus, it is desirable to process the optical film in a manner which permits precise control over the optical properties of the materials used.